The invention relates to improvements in cutting tools in general, and more particularly to improvements in cutting tools which can be used with particular advantage in punching or stamping machines. Still more particularly, the invention relates to improvements in cutting tools of the type wherein a substantially strip-shaped knife is provided with a longitudinally extending cutting edge flanked by two mutually inclined facets.
Cutting tools utilizing strip-shaped knives of steel or the like are disclosed in German Pat. No. 31 35 980. The dimensions of the facets flanking the cutting edge are selected in such a way that the facets penetrate into the material which is being severed so that the material comes into contact with the transition zones between the rearmost portions of the facets and the adjacent foremost portions of the lateral surfaces of the knife. In many instances, the transition zones between the facets and the lateral surfaces are rough or exhibit burrs. Certain knives actually exhibit pronounced beads between the facets and the adjacent foremost portions of the lateral surfaces. Such rough portions or beads are likely to damage and/or deface the material in the region of the cut. Problems are likely to arise when the knife is used to sever workpieces which are made of cardboard and have a thickness in the range of 1 mm. Such materials can consist of kraft pulp. The rough or uneven transition zones between the facets and the lateral surfaces of the knife tend to rip the outermost layers of the workpieces.
Proposals to overcome the just discussed drawbacks of conventional knives include the reduction of the angle between the mutually inclined facets of the knife. However, the angle between the facets cannot be decreased at will because, if the angle is reduced to less than 50 degrees, the knife is subjected to excessive stresses whenever its cutting edge strikes against the workpiece and/or against the anvil on which the workpiece rests. This can considerably reduce the stability and the useful life of the knife.
In accordance with another earlier proposal, the thickness of the strip-shaped knife is increased so that the knife is capable of standing more pronounced stresses even if the angle between the facets is relatively small. However, such knives are likely to deface the workpieces because the tensional stress at the rear side of the workpiece increases proportionately with increasing thickness of the knife. A relatively thick knife is very likely to cause the workpiece (such as a cardboard with two outer layers) to crack so that the product must be discarded.